This invention relates generally to lighting fixtures and, more particularly, to a lighting apparatus suitable for use as part of a lighting fixture, and configured to produce light having a selected color.
Lighting fixtures have been used for many years in theater, television, and architectural lighting applications. Typically, each fixture includes an incandescent lamp mounted adjacent to a concave reflector, which reflects light through a lens assembly to project a beam of light toward a theater stage or the like. A color filter can be mounted at the fixture""s forward end, for transmitting only selected wavelengths of the light emitted by the lamp, while absorbing and/or reflecting other wavelengths. This provides the projected beam with a particular spectral composition.
The color filters used in these lighting fixtures typically have the form of glass or plastic films, e.g., of polyester or polycarbonate, carrying a dispersed chemical dye. The dyes transmit certain wavelengths of light, but absorb the other wavelengths. Several hundred different colors can be provided by such filters, and certain of these colors have been widely accepted as standard colors in the industry.
Although generally effective, such plastic color filters usually have limited lifetimes, caused principally by the need to dissipate large amounts of heat derived from the absorbed wavelengths. This has been a particular problem for filters transmitting blue and green wavelengths. Further, although the variety of colors that can be provided is large, these colors nevertheless are limited by the availability of commercial dyes and the compatibility of those dyes with the glass or plastic substrates. In addition, the very mechanism of absorbing non-selected wavelengths is inherently inefficient. Substantial energy is lost to heat.
In some lighting applications, gas discharge lamps have been substituted for the incandescent lamps, and dichroic filters have been substituted for the color filters. Such dichroic filters typically have the form of a glass substrate carrying a multi-layer dichroic coating, which reflects certain wavelengths and transmits the remaining wavelengths. These alternative lighting fixtures generally have improved efficiency, and their dichroic filters are not subject to fading or other degradation caused by overheating. However, the dichroic filters offer only limited control of color, and the fixtures cannot replicate many of the complex colors created by the absorptive filters that have been accepted as industry standards.
It often is desirable to change the color of the light being produced by a particular lighting fixture, so several remotely operated color-changing devices have been developed in recent years. One such device is a color scroller, which includes a scroll typically containing 16 preselected filters. These filters are subject to the same problems of fading and deformation as are the individual filters. Another such device is a dichroic color wheel, which includes a rotatable wheel carrying about eight preselected dichroic coatings. These color wheels avoid the noted problems of fading and deformation, but are able to carry fewer colors and are substantially more expensive than is a color scroller.
Other such remotely operated color-changing devices include a CMY filter scroller system and a CMY dichroic color mixing system, the latter of which can provide about 16 million combinations of separate colors. However, because both CMY systems use filters that each transmit only about one third of the visible spectrum, they are unable to replicate the spectral nuances of a complex color, including those produced by a conventional color filter in combination with a full-spectrum incandescent light source.
Yet other such remotely operated color-changing devices include an incandescent RGB fixture, such as a theatrical strip light. Such fixtures have similar problems to those of the two CMY systems described briefly above. In such fixtures, one-third of the visible spectrum is provided by each of three separately filtered lid sources. Thus, these fixtures waste two-thirds of the light energy just to project white light, and they waste even more light energy when projecting colored light.
Recently, some lighting fixtures have substituted light-emitting diodes (LEDs) for incandescent lamps and gas-discharge lamps. Equal quantities of red-, green-, and blue-colored LEDs typically have been used, arranged in a suitable array. Some LED fixtures have further included an equal quantity of amber-colored LEDs. By providing electrical power in selected amounts to these LEDs, typically using pulse-width modulated electrical current, light having a variety of colors can be projected. These fixtures eliminate the need for color filters, thereby improving on the efficiency of prior fixtures incorporating incandescent lamps or gas-discharge lamps.
Lighting fixtures incorporating red-, green-, and blue-colored LEDs, i.e., RGB LED fixtures, can project beams of light having an apparent color of white, especially when illuminating a white or other fully reflective surface. However, the actual spectrum of this apparent white color is not at all the same as that of the white light provided by fixtures incorporating incandescent lamps. This is because LEDs emit light in narrow wavelength bands, and merely three different LED colors are insufficient to cover the full visible spectrum. Colored objects illuminated by such RGB LED fixtures frequently do not appear in their true colors. For example, an object that reflects only yellow light, and thus that appears to be yellow when illuminated with white light, will appear black when illuminated with light having an apparent yellow color, produced by the red and green LEDs of an RGB LED fixture. Such fixtures, therefore, are considered to provide poor color rendition when illuminating a setting such as a theater stage, television set, building interior, or display window.
A limited number of LED lighting fixtures have included not only LEDs emitting red, green, and blue light, but also LEDs emitting amber light. Such fixtures are sometimes called RGBA LED fixtures. These fixtures are subject to the same drawbacks as are RGB LED fixtures, but to a slightly reduced degree.
FIG. 1 depicts the luminous flux spectrum of a beam of light projected by a prior art Source Four(copyright) lighting fixture having an incandescent lamp operating at about 3250xc2x0 Kelvin (xc2x0 K) and having no color filter in the beam""s path. The Source Four(copyright) fixture is available from Electronic Theatre Controls, of Middleton, Wis. It will be noted that the spectrum is generally bell-shaped across the visible spectrum, i.e., from about 420 nanometers (nn) to about680 run. The actual radiometric flux spectrum for the light is fairly uniform; however, the depicted luminous flux spectrum is derived by multiplying the radiometric flux spectrum by the spectral sensitivity of the human eye, which is generally bell-shaped. Humans generally perceive the light to be white and are pleased with its appearance.
Also depicted in FIG. 1 is the luminous flux spectrum of a beam of light produced by a prior art RGB LED lighting fixture having equal quantities of red-, green-, and blue-colored LEDs, operating at full power. The two depicted spectra are normalized so that they have approximately equal total flux.
Against a white background or other fully reflective surface, humans will perceive the light produced by the prior art RGB LED lighting fixture, operating at full power, to be somewhat bluish-white. It will be noted in FIG. 1, however, that the actual luminous flux spectrum of such light is highly non-uniform and differs substantially from that of the light produced by the incandescent lamp fixture. This spectral difference can lead to sharp differences in the appearances of many colored objects illuminated by such light.
Integrating the absolute value of the difference between the two spectra depicted in FIG. 1, i.e., the luminous flux spectrum of light produced by an incandescent lamp lighting fixture and the luminous flux spectrum of light produced by an RGB LED lighting fixture, across the visible spectrum, provides a useful measure of conformance between the two spectra. This conformance measure is referred to as a Normalized Mean Deviation (NMD). An NMD of 0% would represent exact conformance between the two spectra. In the particular case of the two spectra depicted in FIG. 1, an NMD of 57.1% is realized. This is considered to be undesirably high, and it indicates that the RGB LED lighting fixture provides a poor emulation of the incandescent lamp lighting fixture and thus provides poor color rendition.
It should be apparent from the foregoing description that there is a need for an improved lighting apparatus, suitable for use as part of a lighting fixture, having individually colored light sources, e.g., LEDs, that improve on the power efficiency of fixtures incorporating incandescent lamps and gas-discharge lamps, yet that can produce beams of light having luminous flux spectra that can be more precisely controlled and, further, that can closely emulate the spectra of prior lighting fixtures and thus provide improved color rendition. The present invention satisfies these needs and provides further related advantages.
One feature of the present invention resides in a lighting apparatus, suitable for use as part of a lighting fixture, for producing a beam of light having a precisely controlled luminous flux spectrum including, for example spectra emulating that of a beam of light produced by a predetermined light source, with or without a color filter. The lighting apparatus includes a plurality of groups of light-emitting devices, each such group configured to emit light having a distinct luminous flux spectrum, with a peak flux wavelength and a predetermined spectral half-width. The spectral half-width of each group is less than about 40 nanometers (nm), and the groups are configured such that the peak flux wavelength of each group is spaced less than about 50 nm from that of another group. The lighting apparatus further includes a controller configurable to supply selected amounts of electrical power to the groups of light-emitting devices, such that the groups cooperate to produce a composite beam of light having a prescribed luminous flux spectrum.
Another feature of the invention resides in a lighting apparatus, suitable for use a part of a lighting fixture, for producing a beam of light having a luminous flux spectrum emulating that of a beam of light produced by a predetermined light source having an incandescent lamp, such light source being free of a filter that modifies the luminous flux spectrum of the light emitted by the lamp. The lighting apparatus includes a plurality of groups of light-emitting devices and further includes a controller configurable to supply selected amounts of electrical power to the groups of light-emitting devices. The groups cooperate to produce a composite beam of light having a prescribed luminous flux spectrum that has a normalized mean deviation across the visible spectrum of less than about 30% relative to the luminous flux spectrum of a beam of light produced by the predetermined light source to be emulated.
More preferably, the luminous flux spectrum of the composite beam of light has a normalized mean deviation across the visible spectrum of less than 25%, and most preferably less than 20%, relative to that of the beam of light to be emulated. In addition, in this feature of the invention, the quantities of devices included in each of the plurality of groups of light-emitting devices can be selected such that, if the controller supplies maximum electrical power to all of the groups, then the resulting composite beam of light will have a luminous flux spectrum having a normalized mean deviation across the visible spectrum of less than about 30% relative to the luminous flux spectrum of a beam of light to be emulated. In addition, the luminous flux spectra of the beam of light produced by the lighting apparatus and of the beam of light produced by the predetermined light source to be emulated preferably are within 5 db of each other across the visible spectrum when the controller supplies prescribed maximum amounts of electrical power to all of the groups of light-emitting devices.
Alternatively, the quantities of devices included in each of the plurality of groups of light-emitting devices are selected such that, if the controller supplies maximum electrical power to all of the groups, then the resulting composite beam of light will have a luminous flux spectrum emulating that of any other prescribed light source. For example, the spectrum of the composite beam of light could be made to have a normalized mean deviation across the visible spectrum of less than about 30% relative to the luminous flux spectrum of a theoretical beam of light produced by a predetermined light source having an incandescent lamp, but modified by a theoretical superposition of the spectral transmissions of a plurality of known color filters.
Another independent feature of the invention resides in a lighting apparatus for producing a beam of colored light having a prescribed luminous flux spectrum, the apparatus including a plurality of groups of light-emitting devices, and further including a controller configurable to supply selected amounts of electrical power to the groups of light-emitting devices, such that they cooperate to produce a composite beam of light having a luminous flux spectrum with substantial energy only within a contiguous bandwidth of less than about 200 nm when the controller supplies prescribed maximum amounts of electrical power to all of the groups of light-emitting devices. More preferably, the flux spectrum has substantial energy only within a contiguous bandwidth of less than about 150 nm. In addition, no portion of the contiguous flux spectrum has an intensity more than 5 db lower, or more preferably 2 db lower, than intensities at wavelengths both above and below it.
More particularly, in this feature of the invention, the controller is configurable to supply selected amounts of electrical power to the groups of light-emitting devices, such that the composite beam of light has a luminous flux spectrum emulating that of a predetermined light source having an incandescent lamp and an associated filter that modifies the luminous flux spectrum of the light emitted by the lamp. The luminous flux spectrum of the composite beam of light has a normalized mean deviation across the visible spectrum of less than about 30% relative to the luminous flux spectrum of a beam of light to be emulated. In one example, the quantities of devices included in each of the groups of light-emitting devices are selected such that, if the controller supplies maximum electrical power to all of the groups, then the resulting composite beam of light will have a luminous flux spectrum having a normalized mean deviation across the visible spectrum of less than about 30% relative to the luminous flux spectrum of a theoretical beam of light produced by the predetermined light source, as modified by a theoretical superposition of the spectral transmissions of a plurality of known color filters.
Further, in this feature of the invention, the groups of light-emitting devices can be configured such that the composite beam of light has a luminous flux spectrum having substantial energy only in wavelengths of less than about 600 nm when the controller supplies prescribed maximum amounts of electrical power to all of the groups of light-emitting devices. Alternatively, the groups of light-emitting devices can be configured such that the composite beam of light has a luminous flux spectrum having substantial energy only in wavelengths of more than about 550 um when the controller supplies prescribed maximum amounts of electrical power to all of the groups of light-emitting devices.
Yet another independent feature of the invention resides in a lighting apparatus for producing a beam of light having a prescribed luminous flux spectrum, wherein at least two of a plurality of groups of light-emitting devices include different quantities of devices. The lighting apparatus further includes a controller configurable to supply selected amounts of electrical power to the groups of light-emitting devices, such that they cooperate to produce a composite beam of light having a prescribed luminous flux spectrum. The specific quantities of devices in each group can be selected to provide certain advantages when the lighting apparatus is used to emulate the luminous flux spectrum provided by a particular light source. For example, the quantities can be selected such that if the controller supplies maximum electrical power to all of the groups, then the resulting composite beam of light will have a luminous flux spectrum closely matching that of the beam of light to be emulated.
Still another independent feature of the invention resides in a lighting apparatus that includes five or more groups of light-emitting devices, and further includes a controller configurable to supply selected amounts of electrical power to the five or more groups of light-emitting devices, such that the groups cooperate to produce a composite beam of light having a prescribed luminous flux spectrum. Preferably the lighting apparatus includes eight or more such groups of light-emitting devices, to facilitate greater control of the luminous flux spectrum of the composite beam of light.
In more detailed features of the invention, the groups of light-emitting devices each include a plurality of light-emitting diodes. In addition, the groups of light-emits devices together can comprise an optical assembly that collects the emitted light and projects the composite beam of the light from the lighting apparatus.